The proposed research involves defining the detailed mechanisms by which thyroid hormones regulate biologic processes in mammalian cells. A Cell Culture system has been developed which is responsive to physiologic concentrations of the thyroid hormones. In this system thyroid hormones induce a three-fold increase in the rate of cell growth, glucose utilization and growth hormone secretion. In addition at identical physiological hormone concentrations prolactin secretion is inhibited in GH1 cell cultures. Studies on the early effects of thyroid hormones on cultured GH1 cells suggest an early effect at the level of DNA transcription. Nuclear receptor sites which appear to mediate the action of the thyroid hormones have been characterized in intact cells, isolated nuclei, and in solubilized nuclear extracts. The nuclear binding activity has characteristics of a non-histone protein and is relatively stable after extracted from nuclei in a soluble form. The initial events in the action of the thyroid hormones will be characterized with respect to early observed changes on nuclear template activity and RNA polymerase. The effect of thyroid hormone with resppct to synthesis on transfer RNA, ribosomal RNA and poly-A rich messenger RNA and hetergenous nuclear RNA will be defined. In addition we hope to purify the nuclear receptor for the thyroid hormones and by reconstitution studies in vitro attempt to define the role of the receptor in regulating nuclear activity. Also with respect to the regulation of a final gene product we also propose to clarify the mechanisms by which thyroid homones regulate growth hormone and prolactin secretion with respect to synthetic and secretory rates and possible alterations in the turnover of these polypeptide hormones. It is hoped that these studies will have the dual effect of clarifying the molecular mechanisms of thyroid hormone action as well as provide basic information concerning the regulatory processes of cell growth and differentiation of normal and neoplastic mammalian cells.